Infectivity of retroviruses carrying IL-2–fusion envelopes. Retroviral infections were performed on proliferating and Go/G1-arrested J3-13 cells. The latter cells were obtained by 48 hours of incubation in medium containing 1% serum, which resulted in 80% of cells being arrested in the Go/G1 cell cycle phase, as shown by propidium iodide staining. Proliferating or quiescent (Go/G1-arrested) J3-13 cells were infected with lacZ retroviral vectors carrying the indicated envelope glycoproteins. The retroviruses, which were harvested in serum-free medium, were diluted in medium containg 1% fetal calf serum and deposited on the cells. After 5 hours of infection, virus-containing media were removed and replaced by fresh media supplemented with 1% fetal calf serum only. Cells were then cultured for 48 hours before X-gal staining. Results are expressed as the percentage (mean ± SD; n = 8) of titers on quiescent cells relative to titers on proliferating J3-13 cells.